Fas ligand (hereinafter referred to as FasL) belongs to the tumor necrosis factor (TNF) family, which currently includes FasL, TNF, lymphotoxin, TRAIL (TNF related apoptosis-inducing ligand) CD40 ligand (CD40L), CD27 ligand (CD27L), CD30 ligand (CD30L), and OX40 ligand (OX40L) (Nagata, Cell, 88, 355-365, 1997; Willey et al., Immunity, 3, 673-682, 1995). Most members of the TNF family except for the α-chain of lymphotoxin are synthesized as type II-membrane proteins. However, soluble forms of FasL, TNF-α, and CD40L can be detected in the culture medium of the cell expressing these molecules, indicating that the TNF family members can be cleaved off from the membrane (cell surface) (Perez et al., Cell, 63, 251-258, 1990; Pietravalle et al., J. Biol. Chem., 271, 5965-5967, 1996b; Tanaka et al., EMBO J., 14, 1129-1135, 1995). Since metalloproteinase inhibitors prevent the shedding of FasL as well as TNF-α, it is thought that a metalloproteinase(s) is responsible for cleaving the membrane-bound FasL and TNF-α to their soluble form (Gearing et al., Nature 370, 555-557, 1994; McGeeham et al., Nature, 370, 558-561, 1994; Mohler et al., Nature, 370, 218-220, 1994; Tanaka et al., Nature Med. 2, 317-322, 1996). Recently, a metalloproteinase that specifically cleaves TNF-α has been identified as a member of the ADAM metalloproteinase family (Black et al., Nature 385, 729-733, 1997; Moss et al., Nature, 385, 733-736, 1997). However, the physiological roles of the shedding of TNF family members from the membrane have not been well characterized.
FasL induces apoptosis by binding to its receptor, Fas which is also called CD95 or APO-1 and which is a member of the TNF receptor family. FasL is predominantly expressed in activated T cells as in the case of natural killer (NK) cells (Arase et al., J. Exp. Med., 181, 1235-1238, 1995; Suda et al., J. Immunol., 154, 3806-3813, 1995; Tanaka et al., Nature Med. 2, 317-322, 1996), whereas Fas is ubiquitously expressed in various cells (French et al., J. Cell. Biol. 335-343, 1996; Leithauser et al., Lab. Invest. 69, 415-429, 1993; Suda et al., J. Immunol., 154, 3806-3813, 1995; Watanabe-Fukunaga et al., J. Immunol., 148, 1274-1279, 1992). Analyses of mice lacking Fas or FasL have indicated that FasL is one of the major effector molecules of cytotoxic T lymphocytes (CTLs), such as CD8-positive T cells and CD4-positive Th1-type T cells (Hanabuchi et al., Proc. Natl. Acad. Sci. USA, 91, 4930-4934, 1994; Suda et al., J. Immunol., 154, 3806-3813, 1995; Vignaux and Golstein, Eur. J. Immunol., 24, 923-927, 1994). The role of CTLs is to remove virally infected cells or cancer cells to prevent the spreading of viruses or cancer cells in animals. However, when this system overfunctions, it causes tissue destruction. The involvement of FasL-induced apoptosis in CTL-mediated autoimmune diseases such as hepatitis, insulin-dependent diabetes, and thyroiditis (Hashimoto's disease) has been demonstrated (Shervonsky et al., Cell 89, 17-24, 1997; Giordano et al., Science, 275, 960-963, 1997; Kondo et al., Nature Med., 3, 409-413, 1997).
As described above, membrane-bound FasL can be cleaved to become a soluble form. The human soluble FasL is functional in inducing apoptosis, at least in mouse WR19L cell transformants that overexpress Fas (Tanaka et al., EMBO J., 14, 1129-1135, 1995). Soluble FasL has been detected at a high level in the serum of human patients suffering from large granular leukemia of the T or NK-type, as well as NK lymphoma (Tanaka et al., Nature Med. 2, 317-322, 1996). Since the patients affected by these leukemias also often suffer from hepatitis and neutropenia, it has been postulated that the soluble FasL may cause systemic tissue damage, as found with TNF. However, when the human recombinant soluble FasL was injected into mice, a large amount of FasL was necessary to exihibit its lethal effect, even though the pretreatment of mice with Propionibacterium Acnes greatly sensitized them to the FasL-induced lethality (Tanaka et al., J. Immunol., 158, 2303-2309, 1997).